The long-term goals of this proposal are to dissect the molecular mechanism(s) by which environmental contaminants, primarily 2, 3, 7, 8, tetrachlorodibenzo-p-dioxin (TCDD), act as tumor promoters. Many of the adverse biological effects of TCDD are thought to occur following its activation of the aryl hydrocarbon receptor (AHR), dimerization of the AHR with its DNA binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) and gene activation by the AHR/ARNT heterodimer. Here, we suggest the existence of an additional mechanism; that TCDD induces nuclear localization of the AHR and its heterodimerization with ARNT that disrupts ARNT from its role in other signaling pathways, i.e., that of the Myc/Max heterodimer. Myc/Max is a well-described regulator of cell proliferation, apoptosis, and differentiation that acts, in part, by upregulation of genes such as p53. We found that 10 ARNT synergistically enhances the ability of Myc to upregulate the p53 promoter, 2) the synergistic actions of ARNT on Myc transactivation occurs via the HLH domain of ARNT, 3) TCDD decreases the mRNA levels of p53 and the ability of Myo to upregulated the p53 promoter. These results, and that of others, form the basis of our hypothesis that ARNT acts synergistically with the Myc/Max heterodimer to upregulate the expression levels of p53. Further, we hypothesize that the addition of TCDD and heterodimerization of ARNT with AHR, removes ARNT form this role, decreases the ability of Myc to regulate the expression levels of p53 and hence, decreases the ability of Myc to induce apoptosis and differentiation. To test this idea, we will determine whether altered expression levels of ARNT significantly impact the ability of Myc to upregulate p53 (Aim 1), and potentiate apoptosis and induce differentiation (Aim 2) in the presence or absence of TCDD (Aim 3). We will then determine whether ARNT is present within DNA binding complexes at the p53 promoter (Aim 4) and identify the protein(s) that interact with ARNT to elicit its synergistic effect on Myc transactivation (Aim 5).